The present invention relates generally to imaging systems and more specifically to a freezing point measurement system and a process for using OCT and OCDR technology.
Optical coherence tomography (OCT) is a new imaging modality. OCT has the ability to perform high resolution, high-sensitivity, cross-sectional imaging of microstructures. The use of OCT has several significant advantages over standard optical imaging techniques and ultrasound. First, OCT can directly measure cross-sectional microstructures on a micron scale. Second, OCT can perform imaging of structures in situ and without contact. Third, imaging can be performed in real time, and, fourth, OCT technology is fiber optically based and can be interfaced with a wide range of medical, microscopic, or industrial applications.
Excellent examples of OCT applications are described in the following U.S. patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 6.191,862, Feb. 20, 2001, Methods and apparatus for high speed longitudinal scanning in imaging systems. Swanson, and
U.S. Pat. No. 6,160,826, Dec. 12, 2000, Method and apparatus for performing optical frequency domain reflectometry, Swanson, and
U.S. Pat. No. 6.157,205, Dec. 5, 2000, Grounding scheme for a high-speed data channel. Swanson, and
U.S. Pat. No. 6,134,003, Oct. 17, 2000, Method and apparatus for performing optical measurements using a fiber optic imaging guidewire, catheter or endoscope, Swanson, and
U.S. Pat. No. 5,777,912, Jul. 7, 1998, Linear phase finite impulse response filter with pre-addition, Leung.
As discussed in the above-cited patents, OCT is analagous to ultrasound B mode imaging, except that it uses light rather than sound and performs imaging by measuring the backscattered intensity of light from a microstructure. OCT produces one, two and three dimensional images by directing an optical beam at an object to be imaged, and measuring backscattered light as the beam is scanned across the object. The OCT image is a gray scale or false color two-dimensional representation of backscattered light intensity in a cross-sectional plane. In medical imaging, the OCT image represents the differential backscattering contrast between different tissue types on a micron scale.
There are a variety of interferometric embodiments for OCT systems. One typical implementation uses a fiber optic coupler for the Michelson interferometer. One of the arms of the interferometer is used to deliver and scan the optic beam on a sample.
In addition to OCT, other techniques can be used. For instance, in optical coherence domain reflectometry (OCDR), a longitudinally scanned reference arm and a broad bandwidth light source are used to create reflectivity profiles of a sample""s optical properties. Samples of OCDR systems are described in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 6,160,826, Dec. 12, 2000, Method and apparatus for performing optical frequency domain reflectometry, Swanson, and
U.S. Pat. No. 5,956,355, Sep. 21, 1999, Method and apparatus for performing optical measurements using a rapidly frequency-tuned laser, Swanson, and
U.S. Pat. No. 5,784,352, Jul. 21, 1998, Apparatus and method for accessing data on multilayered optical media, Swanson, and
U.S. Pat. No. 5,465,147, Nov. 7, 1995, Method and apparatus for acquiring images using a CCD detector array and no transverse scanner, Swanson.
There are a variety of applications where temperature at which a liquid or substance freezes (the freezing point) is an important indicator of substance properties. For instance, the freezing point of milk is a strong function of water content. A variety of industries have developed which supply instrumentation to measure freezing points. Typically they use thermo-electric coolers, super-cooling techniques, and thermisters to measure freezing point. An alternative technique to using thermisters is to use optical techniques which can resolve the phase change of the material. OCDR/OCT is one such technology. One advantage of OCDR/OCT technology is that it can spatially resolve the substance, can be used to probe very small volumes (i.e. nanoliter), and can be inexpensive, and can perform rapid measurements. This application of OCVDR/OCT technology is the focus of the present invention.
Conventional freezing point measuring apparatus typically use super-cooling techniques, thermal electric coolers, and thermisters imbedded in the specimen to measure freezing points. When sample sizes become very small or spatially resolved, optical imaging technology offers an attractive technology option. In particular, OCT and OCDR technology can measure very small volumes (xe2x80x94nanoliter) nondestructively. Such sizes may be too small for conventional thermister measurement approaches. It Is the focus of this invention to apply OCT/OCDR technology to freezing point measurements.